More than 20 years ago, Raso and Stollar published the first formal study expressly aimed at inducing antibodies possessing catalytic activity (Raso and Stollar, Biochemistry 14: 591-599 (1975), Raso and Stollar, J. Amer Chem Soc 95: 1621 (1973), Raso and Stollar, Biochemistry 14: 584-591 (1975)). A transition state enzyme inhibitor was designed, synthesized and used as a hapten to elicit complementary antibody combining sites that would mimic the chosen enzyme active site. A fivefold rate enhancement was achieved for the tyrosine transamination reaction occurring at these antibody sites versus free in solution. This modest acceleration is actually quite significant considering that this result was obtained well before the development of hybridoma technology, so that only heterogeneous populations of affinity-purified rabbit serum antibodies could be used. Thus, the action of a small fraction of catalytic antibodies would have been largely offset by an excess of normal, binding antibodies competing by non-productively sequestering the reactants.
With the emergence of monoclonal antibody techniques, the field of catalytic antibodies has exploded, largely due to recent efforts from the laboratories of Lerner, Benkovic and Schultz (Lerner et al., Science 252: 659-667 (1991)). Homogeneous catalytic antibodies can now be selected, purified and studied in the absence of any competing non-catalytic species. Numerous catalytic antibodies, accelerating a large array of diverse chemical reactions, have been produced within the last several years. In light of the rapid progress since the early pioneering work, it is apparent that the time is now ripe to apply this unique technology to the pressing health problems confronting medical scientists today. However, there are several obstacles which still must be overcome before catalytic antibodies can be realistically considered for general clinical use.
The standard approach for generating catalytic antibodies involves immunizing an animal with a stable analog of the transition state of the reaction to be catalyzed and screening for antibodies that, like enzymes, bind more strongly to the transition state analog than to the corresponding substrate. Like enzymes, those select antibodies have a combining site that is complementary to the 3-D and ionic structure of the transition state analog. In a typical experiment, about half of the monoclonal antibodies raised in response to a transition state analog fall into the category. A small subclass (about 1%) of this category of antibodies actually catalyze the reaction of interest and can be identified by specific assay from among the candidates. Typically, the screening of 100-1,000 monoclonal antibodies will lead to one catalytic antibody.
The first-generation catalytic antibodies obtained in this manner are generally much less catalytically active than the corresponding naturally occurring enzymes. They generally display rate accelerations in the range of 500-300,000-fold while enzymes can provide rate enhancements on the order of 10.sup.5 -10.sup.10 -fold over the uncatalyzed reaction. A main challenge therefore consists of engineering second-generation antibodies with higher catalytic efficiencies. Current approaches to this problem involve either chemical or genetic modification to introduce catalytic groups near the antigen combining site (Benkovic, S. J. Annu. Rev. Biochem. 61: 29-54 (1992)). These approaches depend on some knowledge of the active sites of analogous enzymes and must be applied individually to selected antibodies. This is a slow and labor-intensive process without guaranteed success, considering the low success rate of modifying the specificity or increasing the catalytic efficiency of natural enzymes.
Catalytic antibodies are particularly well suited as a novel treatment entity for cocaine addiction because they efficiently destroy the drug and nullify its stimulatory effect. Encouraging results have been obtained for the use of both conventional and catalytic anti-cocaine antibodies as a potential intervention to alleviate cocaine dependence (Yang et al., Journal of the American Chemical Society 118: 5881 (1996), Landry, D. W. J. A., Scientific American 276: 42 (1997), Landry et al., Science 259: 1899 (1993), Fox, B. S., Drug Alcohol Depend 48: 153 (1997), Fox et al., Nat. Med. 2: 1129 (1996), Landry et al., J Addict Dis 16: 1 (1997)). Presently, anti-cocaine catalytic antibodies are obtained by immunizing mice with a transition state analog antigen and then selecting those rare clones which have the appropriate hydrolytic activity. There are several obstacles which still must be overcome before anti-cocaine antibodies can be realistically considered for general clinical use. The use of murine catalytic antibodies will present problems for the treatment of humans with an addiction to drugs because, as foreign proteins, they will elicit an inhibitory immune response. Moreover, these primary catalytic antibodies obtained by current approaches are inadequate since they usually have very low catalytic activity. Furthermore, because of their transient action, it is unlikely that passively administered anti-cocaine antibodies would be effective for the chronic problem of repeated use, which characterizes drug addiction. These facts indicate the need to develop second generation anti-cocaine antibodies with improved catalytic activities that can be generated by the patients themselves.